Background Body mass index (BMI) is an important risk factor for knee osteoarthritis (OA), but BMI depends only upon height and weight and gives no insight in underlying causal pathways.

Objectives To investigate whether the association of BMI and OA can be explained by the amount and proportion of fat mass (FM) and skeletal muscle mass (SMM), and whether it differs between women and men.

Methods Participants of the NEO (Netherlands Epidemiology of Obesity) study, a population-based cohort of individuals aged 45-65 years with a BMI ≥27 kg/m², were used. BMI was assessed by measured weight and length. FM and SMM were assessed using bioelectrical impedance analysis. MR imaging of the right knee was performed on 1.5T (Philips, Best, The Netherlands) using a standard knee protocol. Osteophytes were scored according to the Knee Osteoarthritis Score System in nine compartments. Osteophytes were graded from 0 (absent) to 3 (severe). A total score was calculated for each individual, a score of at least 5 was considered as OA. Associations of BMI, FM and SMM with OA were analyzed using logistic regression analyses, stratified for sex and adjusted for age.

Results After exclusion of participants with known rheumatic diseases other than OA (n=42), 487 participants were analyzed. Median age was 56 years and 54% was female. Median BMI was 30.3 kg/m² (IQR 28.5-33.3), FM 33.6 kg (IQR 27.6-42.2) and SMM 28.0 kg (IQR 22.9-34.2). As expected, BMI, FM and SMM were strongly correlated. OA was present in 39% of individuals. In both women and men, BMI was associated with OA, OR 1.09 (95% CI 1.02-1.15) and OR 1.12 (95% CI 1.02-1.22) respectively. FM was associated in both sexes as well, OR 1.04 (95% CI 1.02-1.07) and OR 1.05 (95% CI 1.01-1.08) respectively. SMM was associated with OA in women only, OR 1.14 (95% CI 1.05-1.24). In a multivariate analysis with BMI, FM and SMM in women, the OR for SMM changed to 1.09 (95% CI 1.00-1.94); the associations with BMI and FM were not significant in this analysis. In the multivariate analysis in men, no significant association of BMI, FM or SMM with OA was found.

Conclusions BMI and FM were associated with OA in both women and men. Multivariate analysis suggests that FM is mediating the association of BMI with OA. In women, SMM was associated with OA independently of BMI and FM, suggesting that SMM has an additional effect on OA in women.

Disclosure of Interest W. Visser Grant/Research support from: Dutch Arthritis Association, Leiden University, Leiden University Medical Center, M. den Heijer: None Declared, M. Reijnierse: None Declared, R. de Mutsert: None Declared, F. Rosendaal: None Declared, M. Kloppenburg: None Declared